The present invention relates generally to motor circuits, and more particularly, to motor circuits that employ pulse width modulation.
Speed control in electric motor devices may be accomplished in a variety of maimers. In brush-type motors that use a DC power source, a semiconductor such as a MOSFET may be used as a resistive load that controls the current provided to the armature windings. The resistance of the MOSFET may be controlled to achieve the desired speed. However, the use of the MOSFET as a series resistive element results in reduced efficiency of the motor.
Another speed control method for brush-type motors employs switching semiconductors. According to this method, semiconductor switches repeatedly open and close the circuit between the armature winding or coil and the DC power supply at a select frequency to drive the motor with a controlled average voltage. One may alter or control the speed in such motors by controlling the semiconductors with a pulse-width modulated signal. The pulse-width modulated (PWM) signal determines the duration that the semiconductor is turned on, or duty cycle, which in turn affects the average voltage provided to the armature windings and thus the speed of the motor.
Both high frequency and low frequency PWM signals have been used to drive DC-powered brush-type motors. Low frequency PWM signals are PWM signals having a switching frequency in the audible frequency range. A drawback to using low frequency PWM signals is that such signals can generate mechanical vibration in the audible range. Such mechanical vibration may be significant in applications in which the motor drives a mechanical assembly, such as for example, a vehicular cooling fan. Mechanical vibration in a vehicular cooling fan can cause an undesirable amount of audible noise.
High frequency PWM signals include ultrasonic or near ultrasonic switching frequencies, for example, 18 kHz and higher. One benefit of using such high frequency PWM signals for motor drives is that such signals typically do not generate audible mechanical vibration. However, one drawback of the use of ultrasonic PWM signals to drive a DC brush-type motor is that such signals can generate a high level of conducted and radiated (and conducted) noise in the RF range. One disadvantage associated with the high level of conducted and radiated noise is that it may require shielding to pass various industry and government requirements for EMI emissions. In general, higher frequencies require more elaborate shielding than lower frequencies.
Accordingly, there exists a need for a DC brush-type motor control circuit that includes the reduce loss benefits of using a switch driven by a PWM signal without the mechanical noise drawbacks of the low frequency PWM signal or the radiated noise drawbacks of the high frequency PWM signals.
In other types of motors, it has been found that randomly, or pseudorandomly, varying the switching frequency of a constant duty cycle PWM signal has the effect of spreading the noise associated with the switching operation. For example, a 6 KHz switching frequency may be varied five or ten percent to spread the RF and/or mechanical noise spectra, thereby reducing the effects of the noise at any one particular frequency. Such technology has been implemented in the context of three-phase asynchronous motors.
A drawback to the random variation of the switching frequency is that it is not easily implemented in typical applications. In particular, PWM signals for motor control use are typically generated by counter circuits that operate off of a clock signal. Counter circuits obtain the clock signal, which is typically several times that of the desired switching frequency. The counter circuits include xe2x80x9cdivide downxe2x80x9d circuitry that divides the clock signal frequency down to the desired switching frequency. Thus, in order to change the switching frequency, the counter must alter the divisor value in its divide down circuitry. Achieving a pseudorandom distribution of frequencies within five percent of the nominal frequency cannot be easily accomplished through such alteration of the divide down circuitry of the counter.
There exists a need, therefore, for a method and apparatus for driving a motor with a PWM signal that has reduced noise and furthermore is readily adapted for use in connection with commonly available and relatively low cost PWM signal generating architectures. There also exists a need for a method and apparatus for driving a DC brush-type motor with a PWM signal that has reduced mechanical and/or RF noise.
The present invention fulfills the above needs, as well as others, by providing a motor control circuit that employs PWM signals having a varying duty cycle. The varying duty cycle spreads the spectrum of the noise associated with the use of PWM signals for controlling the current through the motor windings.
An exemplary embodiment of the present invention is a motor control circuit for use in a motor circuit, the motor control circuit including a varying duty cycle pulse width modulation circuit and a switch. The varying duty cycle pulse width modulation circuit is operable to generate a PWM signal having a varying duty cycle, wherein an average of said varying duty cycle corresponds to a desired motor speed. The switch has a control input operably coupled to the pulse width modulation circuit to receive the PWM signal therefrom. The switch further includes first and second terminals, the first terminal adapted to be coupled to a coil of the motor circuit, the switch operable to selectively electrically connect and disconnect the first and second terminals based in part on the PWM signal received by the control input.
An exemplary method according to the present invention is a method of controlling a motor circuit. The method includes a step generating a PWM signal, the PWM signal having a varying duty cycle, wherein an average of said varying duty cycle corresponds to a desired motor speed. The PWM signal is then provided to a switch. The switch is then employed to alternately electrically connect and disconnect a motor winding to a DC power source based in part on the PWM signal received by the control input.
By using a varying duty cycle PWM signal to switch the coil of the motor circuit, the audible and/or RF noise energy that may otherwise be caused by the switching is spread over a wider spectrum, thereby limiting the concentration of noise energy at any one frequency. In one embodiment of the invention, the spreading of the audible noise spectrum advantageously facilitates the use of low frequency PWM signals for controlling a motor circuit, thereby avoiding the EMI shielding problems associated with the use of higher frequency PWM signals. By varying the duty cycle of the PWM signal, a well distributed noise energy spectrum may be achieved with commonly-used, relatively low cost PWM signal generating circuit components.
The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.